JP2008128080A - Control device for internal combustion engine - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To reduce the drop of estimation accuracy of air fuel ratio for each cylinder due to drop of response of an air fuel ratio sensor in a system estimating air fuel ratio for each cylinder based on a detection value of the air fuel ratio sensor installed on an exhaust gas merging part of an engine. <P>SOLUTION: When response of the air fuel ratio sensor 37 is in a normal range (a condition where response of the air fuel ratio sensor 37 drops very little), a model method is selected as a method for estimating an air fuel ratio for each cylinder and air fuel ratio for each cylinder is estimated by using a model correlating air fuel ratio for each cylinder to detection the value of the air fuel ratio sensor 37. On the other hand, when response of the air fuel ratio sensor 37 drops lower than the normal range, the method for estimating the air fuel ratio for each cylinder is changed over to a dither method which is an estimation method in which influence of drop of response of the air fuel ratio sensor 37 is small and air fuel ratio for each cylinder is estimated based on output of the air fuel ratio sensor 37 when air fuel ratio dither control forcibly changing air fuel ratio for each cylinder is executed. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a control device for an internal combustion engine that performs cylinder-by-cylinder air-fuel ratio estimation that estimates an air-fuel ratio of each cylinder based on a detection value of an air-fuel ratio sensor installed in an exhaust gas merging portion of the internal combustion engine.

  In recent years, in order to improve the air-fuel ratio control accuracy of an internal combustion engine, for example, as described in Patent Document 1 (Japanese Patent No. 2684011) and Patent Document 2 (Japanese Patent Laid-Open No. 2005-207405), a plurality of Estimate the air-fuel ratio of each cylinder using a model that correlates the detection value of one air-fuel ratio sensor (air-fuel ratio of the exhaust gas merger) installed in the exhaust gas merger where the exhaust gas from the cylinders merges with the air-fuel ratio of each cylinder The cylinder-by-cylinder air-fuel ratio estimation is performed, and the air-fuel ratio correction amount is calculated for each cylinder based on the estimation result of the cylinder-by-cylinder air-fuel ratio so that the variation in the air-fuel ratio of each cylinder is reduced. Some cylinder-by-cylinder air-fuel ratio control is performed to control the air-fuel ratio (fuel injection amount) of each cylinder based on the air-fuel ratio correction amount for each cylinder.

Further, in Patent Document 1 (Japanese Patent No. 2684011), it is determined whether or not the air-fuel ratio correction amount for each cylinder calculated based on the estimation result of the cylinder-by-cylinder air-fuel ratio is within a predetermined range. When the air-fuel ratio correction amount of the cylinder exceeds a predetermined range, an abnormality diagnosis for each cylinder is performed to determine that an abnormality has occurred in that cylinder.
Japanese Patent No. 2684011 JP 2005-207405 A

  By the way, the air-fuel ratio of each cylinder using a model in which the detection value of one air-fuel ratio sensor installed in the exhaust gas merging portion is associated with the air-fuel ratio of each cylinder, as in the techniques of Patent Document 1 and Patent Document 2. In the cylinder-by-cylinder air-fuel ratio estimation that estimates the air-fuel ratio, if the responsiveness of the air-fuel ratio sensor decreases due to deterioration over time or the like, the estimation accuracy of the cylinder-by-cylinder air-fuel ratio estimation may decrease. When the estimation accuracy of the cylinder-by-cylinder air-fuel ratio estimation decreases, the diagnosis accuracy of the cylinder-by-cylinder abnormality diagnosis using the estimation result of the cylinder-by-cylinder air-fuel ratio estimation and the control accuracy of the cylinder-by-cylinder air-fuel ratio control may decrease.

  The present invention has been made in consideration of such circumstances. Therefore, the object of the present invention is to suppress a decrease in estimation accuracy of cylinder-by-cylinder air-fuel ratio estimation when the response of the air-fuel ratio sensor is lowered. Another object of the present invention is to provide a control device for an internal combustion engine that can reduce adverse effects caused by a decrease in responsiveness of an air-fuel ratio sensor.

  In order to achieve the above object, according to the first aspect of the present invention, an air-fuel ratio sensor for detecting an air-fuel ratio of the exhaust gas is installed at an exhaust confluence where the exhaust gases of a plurality of cylinders of the internal combustion engine merge. In a control apparatus for an internal combustion engine having a cylinder-by-cylinder air-fuel ratio estimating means for estimating an air-fuel ratio of each cylinder based on a detection value of an air-fuel ratio sensor, the response of the air-fuel ratio sensor is responsiveness detecting means. The cylinder-by-cylinder air-fuel ratio estimation means has a plurality of types of cylinder-by-cylinder air-fuel ratio estimation methods for estimating the air-fuel ratio of each cylinder based on the detection value of the air-fuel ratio sensor, and is detected by the responsiveness detection means. The cylinder-by-cylinder air-fuel ratio estimation method is switched according to the response of the air-fuel ratio sensor.

  In this configuration, when the responsiveness of the air-fuel ratio sensor decreases due to deterioration over time or the like, the cylinder-by-cylinder air-fuel ratio estimation estimation method can be switched to an estimation method that is less affected by a decrease in the responsiveness of the air-fuel ratio sensor. Even when the responsiveness of the air-fuel ratio sensor is lowered, it is possible to reduce a decrease in the estimation accuracy of the cylinder-by-cylinder air-fuel ratio, and it is possible to reduce an adverse effect due to the lowered responsiveness of the air-fuel ratio sensor.

  In this case, when the responsiveness of the air-fuel ratio sensor is within a predetermined normal range as in claim 2, the cylinder-by-cylinder air-fuel ratio estimation method associates the detected value of the air-fuel ratio sensor with the air-fuel ratio of each cylinder. It is preferable to select a model method for estimating the air-fuel ratio of each cylinder using the model. In this way, when the responsiveness of the air-fuel ratio sensor is within the normal range (a state in which the responsiveness of the air-fuel ratio sensor has hardly decreased), the detected value of the air-fuel ratio sensor and the air-fuel ratio of each cylinder are The air-fuel ratio of each cylinder can be accurately estimated by a model method that estimates the air-fuel ratio of each cylinder using the associated model. In addition, since the dither method described later executes air-fuel ratio dither control that forcibly changes the air-fuel ratio for each cylinder, drivability and exhaust emission may be reduced. Since it is not necessary to forcibly change, it is possible to estimate the air-fuel ratio of each cylinder without causing deterioration in drivability and exhaust emission.

  Further, as described in claim 3, when the responsiveness of the air-fuel ratio sensor falls below a predetermined normal range, an air-fuel ratio for forcibly changing the air-fuel ratio for each cylinder is used as a cylinder-by-cylinder air-fuel ratio estimation method. It is preferable to select a dither method for estimating the air-fuel ratio of each cylinder based on the output of the air-fuel ratio sensor when the dither control is executed. Since the dither method is an estimation method that is less affected by a decrease in the response of the air-fuel ratio sensor than the model method, the dither method is selected when the response of the air-fuel ratio sensor falls below the normal range. If the air-fuel ratio of each cylinder is estimated, the air-fuel ratio of each cylinder can be estimated more accurately than the model method, and the estimation accuracy of the cylinder-by-cylinder air-fuel ratio can be estimated even when the responsiveness of the air-fuel ratio sensor is reduced. Can be reduced.

  By the way, when the air-fuel ratio of each cylinder is estimated by the dither method, if the air-fuel ratio change amount when the air-fuel ratio is forcibly changed by the air-fuel ratio dither control is increased, the estimation accuracy of the cylinder-by-cylinder air-fuel ratio increases. However, on the other hand, there is a concern that the adverse effect on drivability and exhaust emissions will increase.

  Therefore, when the dither method is selected as the cylinder-by-cylinder air-fuel ratio estimation method as in claim 4, the air-fuel ratio is forcibly changed by the air-fuel ratio dither control according to the degree of decrease in the response of the air-fuel ratio sensor. The air-fuel ratio change amount at that time may be set. In this way, the air-fuel ratio change amount of the air-fuel ratio dither control is changed in accordance with the degree of decrease in the responsiveness of the air-fuel ratio sensor, and the drivability and exhaust gas are exhausted while adequately estimating the cylinder-by-cylinder air-fuel ratio estimation accuracy. The air-fuel ratio change amount of the air-fuel ratio dither control can be set so as to minimize the adverse effect on the emission.

  In addition, the present invention is preferably applied to a system for performing an abnormality diagnosis for each cylinder that determines whether or not there is an abnormality in each cylinder based on the estimation result of the air-fuel ratio for each cylinder. In this way, even when the responsiveness of the air-fuel ratio sensor is lowered, the deterioration of the estimation accuracy of the cylinder-by-cylinder air-fuel ratio is suppressed, and the diagnosis accuracy of the cylinder-by-cylinder abnormality diagnosis using the estimation result of the cylinder-by-cylinder air-fuel ratio is improved. The decrease can be suppressed.

Hereinafter, an embodiment embodying the best mode for carrying out the present invention will be described.
First, a schematic configuration of the entire engine control system will be described with reference to FIG.
An air cleaner 13 is provided at the most upstream portion of the intake pipe 12 of an in-line four-cylinder engine 11 that is an internal combustion engine, for example, and an air flow meter 14 that detects the intake air amount is provided downstream of the air cleaner 13. . On the downstream side of the air flow meter 14, a throttle valve 15 whose opening is adjusted by a motor or the like and a throttle opening sensor 16 for detecting the throttle opening are provided.

  Further, a surge tank 17 is provided on the downstream side of the throttle valve 15, and an intake pipe pressure sensor 18 for detecting the intake pipe pressure is provided in the surge tank 17. The surge tank 17 is provided with an intake manifold 19 for introducing air into each cylinder of the engine 11, and a fuel injection valve 20 for injecting fuel is attached in the vicinity of the intake port of the intake manifold 19 of each cylinder. Yes. During engine operation, the fuel in the fuel tank 21 is sent to the delivery pipe 23 by the fuel pump 22 and fuel is injected from the fuel injection valve 20 of each cylinder at each injection timing of each cylinder. A fuel pressure sensor 24 that detects fuel pressure (fuel pressure) is attached to the delivery pipe 23.

  Further, the engine 11 is provided with variable valve timing mechanisms 27 and 28 for changing the opening and closing timings of the intake valve 25 and the exhaust valve 26, respectively. Further, the engine 11 is provided with an intake cam angle sensor 31 and an exhaust cam angle sensor 32 that output a cam angle signal in synchronization with the rotation of the intake cam shaft 29 and the exhaust cam shaft 30, and the crank of the engine 11. A crank angle sensor 33 that outputs a pulse of a crank angle signal at every predetermined crank angle (for example, every 30 ° C. A) in synchronization with the rotation of the shaft is provided.

  On the other hand, an air-fuel ratio sensor 37 for detecting the air-fuel ratio of the exhaust gas is installed in the exhaust gas converging portion 36 where the exhaust manifold 35 of each cylinder of the engine 11 joins. A catalyst 38 such as a three-way catalyst for purifying CO, HC, NOx and the like is provided.

  Outputs of various sensors such as the air-fuel ratio sensor 37 described above are input to an engine control circuit (hereinafter referred to as “ECU”) 40. The ECU 40 is mainly composed of a microcomputer, and executes various engine control programs stored in a built-in ROM (storage medium), so that the fuel of the fuel injection valve 20 of each cylinder according to the engine operating state. Control injection quantity and ignition timing.

  Further, the ECU 40 executes a routine shown in FIGS. 3 to 6 to be described later, thereby performing cylinder-by-cylinder air-fuel ratio estimation for estimating the air-fuel ratio of each cylinder based on the detected value of the air-fuel ratio sensor 37. Or a “dither method” to be described later, and abnormality diagnosis for each cylinder is performed to determine whether each cylinder is abnormal based on the estimation result of the air-fuel ratio for each cylinder.

Here, the model method and the dither method will be described.
The model method uses a model in which the detection value of the air-fuel ratio sensor 37 (detected air-fuel ratio of exhaust gas flowing through the exhaust gas merging portion 36) is associated with the air-fuel ratio of each cylinder (hereinafter referred to as “cylinder-specific air-fuel ratio estimation model”). Thus, the air-fuel ratio of each cylinder is estimated.

  Specifically, paying attention to the gas exchange in the exhaust gas merging portion 36, the detected value of the air-fuel ratio sensor 37 is changed to the estimated air-fuel ratio history of each cylinder and the detected value history of the air-fuel ratio sensor 37 in the exhaust gas merging portion 36. Are respectively multiplied by predetermined weights and added, and the air-fuel ratio of each cylinder is estimated using the model. At this time, a Kalman filter is used as an observer.

More specifically, a gas exchange model in the exhaust merging portion 36 is approximated by the following equation (1).
ys (t) = k1 * u (t-1) + k2 * u (t-2) -k3 * ys (t-1) -k4 * ys (t-2)
...... (1)
Here, ys is a detected value of the air-fuel ratio sensor 37, u is an air-fuel ratio of the gas flowing into the exhaust merging section 36, and k1 to k4 are constants.

  In the exhaust system, there are a primary delay element of gas inflow and mixing in the exhaust confluence 36 and a primary delay element due to a response delay of the air-fuel ratio sensor 37. Therefore, in the above equation (1), the history for the past two times is referred to in consideration of these first order lag elements.

When the above equation (1) is converted into a state space model, the following equations (2a) and (2b) are derived.
X (t + 1) = A.X (t) + B.u (t) + W (t) (2a)
Y (t) = C · X (t) + D · u (t) (2b)
Here, A, B, C, and D are model parameters, Y is a detected value of the air-fuel ratio sensor 37, X is an estimated air-fuel ratio of each cylinder as a state variable, and W is noise.

Further, when the Kalman filter is designed by the above equations (2a) and (2b), the following equation (3) is obtained.
X ^ (k + 1 | k) = A.X ^ (k | k-1) + K {Y (k) -C.A.X ^ (k | k-1)} (3) where X ^ (X hat) is the estimated air-fuel ratio of each cylinder, and K is the Kalman gain. The meaning of X ^ (k + 1 | k) represents that the estimated value of the next time (k + 1) is obtained from the estimated value of time (k).

  As described above, the cylinder-by-cylinder air-fuel ratio estimation model is configured by the Kalman filter type observer, whereby the air-fuel ratio of each cylinder can be sequentially estimated as the combustion cycle proceeds.

  On the other hand, the dither method executes air-fuel ratio dither control for forcibly changing the air-fuel ratio for each cylinder, and the air-fuel ratio of each cylinder is based on the output of the air-fuel ratio sensor 37 when this air-fuel ratio dither control is executed. Is a method of estimating

  Specifically, as shown in FIG. 2 (a), before the start of the air-fuel ratio dither control in the i-th cylinder #i (i = 1 to 4 in the case of a four-cylinder engine) that is the current air-fuel ratio estimation target. By calculating the deviation Y1 (#i) between the detected air-fuel ratio of the i-th cylinder #i detected by the air-fuel ratio sensor 37 and the reference air-fuel ratio (before forcibly changing the air-fuel ratio), the air-fuel ratio dither control is performed. The inter-cylinder deviation Y1 (#i) of the detected air-fuel ratio of the i-th cylinder #i before the start of is determined. Here, the reference air-fuel ratio is set to the average value of the detected air-fuel ratios of all the cylinders detected by the air-fuel ratio sensor 37 before the start of the air-fuel ratio dither control. Alternatively, the reference air-fuel ratio may be set to a predetermined fixed value (for example, 14.7).

  Thereafter, air-fuel ratio dither control for forcibly changing the air-fuel ratio of the i-th cylinder #i by a predetermined change amount ΔX (#i) in the rich direction or the lean direction is executed, as shown in FIG. Calculate the deviation Y2 (#i) between the detected air-fuel ratio of the i-th cylinder #i detected by the air-fuel ratio sensor 37 and the reference air-fuel ratio after starting the air-fuel ratio dither control (after forcibly changing the air-fuel ratio) Thus, the inter-cylinder deviation Y2 (#i) of the detected air-fuel ratio of the i-th cylinder #i after the start of the air-fuel ratio dither control is obtained.

Thereafter, the actual air-fuel ratio change ΔX (#i) when the air-fuel ratio (for example, fuel injection amount) of the i-th cylinder #i is forcibly changed by the air-fuel ratio dither control, and the air-fuel ratio at that time The change amount ΔY (#i) {= Y2 (#i) −Y1 (#i)} of the air-fuel ratio detected by the sensor 37 and the inter-cylinder deviation of the air-fuel ratio detected by the air-fuel ratio sensor 37 before the start of the air-fuel ratio dither control Using Y1 (#i), the actual air-fuel ratio deviation X (#i) before starting the air-fuel ratio dither control is obtained by the following equation.
X (#i) = ΔX (#i) × Y1 (#i) / ΔY (#i)
= ΔX (#i) × Y1 (#i) / {Y2 (#i) −Y1 (#i)}

  Based on the inter-cylinder deviation X (#i) of the actual air-fuel ratio of the i-th cylinder #i obtained in this way and the reference air-fuel ratio (the average value or the control target value of the estimated air-fuel ratio of all the cylinders). By calculating the air-fuel ratio of cylinder #i, the air-fuel ratio of each cylinder can be estimated sequentially as the combustion cycle progresses.

  By the way, in the cylinder-by-cylinder air-fuel ratio estimation based on the above-described model method, if the responsiveness of the air-fuel ratio sensor 37 decreases due to deterioration over time or the like, the estimation accuracy of the cylinder-by-cylinder air-fuel ratio may be lowered. When the accuracy is lowered, there is a possibility that the diagnosis accuracy of the abnormality diagnosis for each cylinder using the estimation result of the air-fuel ratio for each cylinder is lowered.

  Therefore, in this embodiment, the response of the air-fuel ratio sensor 37 is detected, and the cylinder-by-cylinder air-fuel ratio estimation method is switched between the model method and the dither method in accordance with the response of the air-fuel ratio sensor 37.

  Specifically, when the responsiveness of the air-fuel ratio sensor 37 is within a predetermined normal range (a state in which the responsiveness of the air-fuel ratio sensor 37 has hardly decreased), the model method is selected as the cylinder-by-cylinder air-fuel ratio estimation method. To do. Thus, when the responsiveness of the air-fuel ratio sensor 37 is within the normal range, the air-fuel ratio of each cylinder is accurately estimated by the model method that estimates the air-fuel ratio of each cylinder using the cylinder-by-cylinder air-fuel ratio estimation model. To do.

  On the other hand, when the responsiveness of the air-fuel ratio sensor 37 falls below the normal range, the dither method is selected as the cylinder-by-cylinder air-fuel ratio estimation method. Since the dither method is an estimation method that is less affected by a decrease in the response of the air-fuel ratio sensor 37 than the model method, the dither method is selected when the response of the air-fuel ratio sensor 37 is lower than the normal range. If the air-fuel ratio of each cylinder is estimated, the air-fuel ratio of each cylinder can be estimated with higher accuracy than the model method.

  When estimating the air-fuel ratio of each cylinder by this dither method, if the air-fuel ratio change amount when forcibly changing the air-fuel ratio by the air-fuel ratio dither control is increased, the estimation accuracy of the cylinder-by-cylinder air-fuel ratio increases. On the other hand, there is a concern that adverse effects on drivability and exhaust emissions will increase.

  Therefore, in the present embodiment, when the dither method is selected as the cylinder-by-cylinder air-fuel ratio estimation method, the air-fuel ratio change amount of the air-fuel ratio dither control is changed according to the degree of decrease in the responsiveness of the air-fuel ratio sensor 37, The air-fuel ratio change amount of the air-fuel ratio dither control is set so as to minimize the adverse effect on drivability and exhaust emission while appropriately ensuring the estimation accuracy of the cylinder-by-cylinder air-fuel ratio.

  The above-described cylinder-by-cylinder air-fuel ratio estimation, cylinder-by-cylinder abnormality diagnosis, and the like are executed by the ECU 40 according to the routines shown in FIGS. The processing contents of each routine will be described below.

[Sensor abnormality diagnosis routine]
The sensor abnormality diagnosis routine shown in FIG. 3 is executed at a predetermined cycle while the ECU 40 is turned on. When this routine is started, first, at step 101, it is determined whether or not a fuel cut has been started. If the fuel cut has not been started, this routine is terminated without executing the processing from step 102 onward. To do.

  Thereafter, when it is determined in step 101 that the fuel cut has been started, the routine proceeds to step 102 where the output I1 of the air-fuel ratio sensor 37 at the start of the fuel cut is read and stored in the memory of the ECU 40 and the timer is set. Operate and measure the elapsed time from the start of fuel cut.

  Thereafter, the routine proceeds to step 103, where it is determined whether or not the output of the air-fuel ratio sensor 37 has changed to a predetermined value I2, and when it is determined that the output of the air-fuel ratio sensor 37 has changed to a predetermined value I2, step 104 is performed. Then, based on the count value of the timer, the response time T1 required from the start of the fuel cut until the output of the air-fuel ratio sensor 37 changes to the predetermined value I2 is measured.

  Thereafter, the routine proceeds to step 105, where the response time T1 of the air-fuel ratio sensor 37 is converted into a responsiveness index Rs. In this case, for example, by setting the reciprocal of the response time T1 as the response index Rs, the response index Rs is set to be larger as the response of the air-fuel ratio sensor 37 is higher (that is, the response time T1 is shorter). This responsiveness index Rs is used when determining the responsiveness of the air-fuel ratio sensor 37 in the cylinder-specific abnormality diagnosis routine of FIGS. The process of step 105 plays a role as responsiveness detecting means in the claims.

Thereafter, the routine proceeds to step 106, where the output change rate ΔI of the air-fuel ratio sensor 37 is calculated from the following equation.
ΔI = (I2 -I1) / T1
The output change rate ΔI may be used as the response index Rs.

Thereafter, the routine proceeds to step 107, where it is determined whether or not the output change rate ΔI of the air-fuel ratio sensor 37 is smaller than a predetermined abnormality determination value Ifc.
As a result, when it is determined that the output change rate ΔI of the air-fuel ratio sensor 37 is smaller than the abnormality determination value Ifc, it is determined that there is an abnormality in the air-fuel ratio sensor 37 and the routine proceeds to step 108 where the air-fuel ratio sensor 37 The abnormality flag is set to "1" and a warning lamp (not shown) provided on the instrument panel of the driver's seat is turned on, or a warning display section (not shown) of the instrument panel of the driver's seat The warning information is displayed to warn the driver, and the abnormality information (abnormality code or the like) is stored in a rewritable nonvolatile memory such as a backup RAM (not shown) of the ECU 40, and this routine is terminated.

  In contrast, if it is determined in step 107 that the output change rate ΔI of the air-fuel ratio sensor 37 is equal to or greater than the abnormality determination value Ifc, it is determined that the air-fuel ratio sensor 37 is not abnormal (normal), This routine ends.

[Cylinder-specific abnormality diagnosis routine]
The abnormality diagnosis routine for each cylinder shown in FIGS. 4 and 5 is executed at a predetermined cycle while the ECU 40 is turned on, and serves as a cylinder abnormality diagnosis means in the scope of claims. When this routine is started, first, in step 201, it is determined whether or not another abnormality (for example, abnormality of the fuel injection valve or abnormality of the fuel pump) that affects the responsiveness of the air-fuel ratio has occurred. To do.

  If it is determined in step 201 that another abnormality that affects the responsiveness of the air-fuel ratio has occurred, it is determined that there is a possibility that the diagnosis accuracy of the cylinder-by-cylinder abnormality diagnosis may be reduced. This routine is terminated without executing the processing from 202 onward.

  On the other hand, if it is determined in step 201 that no other abnormality affecting the air-fuel ratio responsiveness has occurred, the process proceeds to step 202, where the engine speed, engine load (intake air amount and intake pipe) are increased. After reading the engine operating state such as (pressure), the process proceeds to step 203 to determine whether or not the current engine operating state is a predetermined region. Here, the predetermined region is an operation region in which the estimation accuracy of the cylinder-by-cylinder air-fuel ratio based on the detection value of the air-fuel ratio sensor 37 is increased, and is set to, for example, a low rotation and high load region.

  If it is determined in step 203 that the current engine operating state is not in the predetermined region, it is determined that there is a possibility that the diagnosis accuracy of the cylinder-by-cylinder abnormality diagnosis using the estimation result of the cylinder-by-cylinder air-fuel ratio may be lowered. Thus, this routine is terminated without executing the processing from step 204 onward.

  On the other hand, if it is determined in step 203 that the current engine operating state is in the predetermined region, it is determined that the diagnosis accuracy of the cylinder-by-cylinder abnormality diagnosis using the estimation result of the cylinder-by-cylinder air-fuel ratio can be ensured, The processing after step 204 is executed as follows.

  First, in step 204, the diagnosis execution flag is set to "1", and then the process proceeds to step 205, in which the responsiveness index Rs of the air-fuel ratio sensor 37 calculated in the sensor abnormality diagnosis routine of FIG. 3 is the first determination value Rr1. It is determined whether or not the responsiveness of the air-fuel ratio sensor 37 is within the normal range (a state in which the responsiveness of the air-fuel ratio sensor 37 has hardly deteriorated).

  When it is determined in step 205 that the response index Rs of the air-fuel ratio sensor 37 is larger than the first determination value Rr1 (when the response of the air-fuel ratio sensor 37 is determined to be within the normal range). Advances to step 206, selects a model method as the cylinder-by-cylinder air-fuel ratio estimation method, executes a cylinder-by-cylinder air-fuel ratio estimation routine (not shown), and uses the cylinder-by-cylinder air-fuel ratio estimation model to The air-fuel ratio of the i-th cylinder #i cylinder to be estimated is estimated.

  On the other hand, if it is determined in step 205 that the responsiveness index Rs of the air-fuel ratio sensor 37 is equal to or less than the first determination value Rr1 (determined that the responsiveness of the air-fuel ratio sensor 37 is lower than the normal range). In the case where the dither method is selected as the cylinder-by-cylinder air-fuel ratio estimation method. In this case, in the next steps 207 and 208, the air-fuel ratio sensor 37 is compared with the second determination value Rr2 or the third determination value Rr3 (where Rr1> Rr2> Rr3). After determining the degree of responsiveness decrease of 37, in steps 209 to 211, the air-fuel ratio change amount ΔX of the air-fuel ratio dither control is changed into three stages of change amounts ΔX1 to ΔX3 in accordance with the degree of responsiveness decrease of the air-fuel ratio sensor 37. (However, one of ΔX1 <ΔX2 <ΔX3) is set.

  First, at step 207, it is determined whether or not the responsiveness index Rs of the air-fuel ratio sensor 37 is larger than the second determination value Rr2, and the responsiveness index Rs of the air-fuel ratio sensor 37 is larger than the second determination value Rr2. If it is determined that it is large, it is determined that the degree of decrease in the responsiveness of the air-fuel ratio sensor 37 is small, and the routine proceeds to step 209 where the air-fuel ratio change amount ΔX of the air-fuel ratio dither control is the smallest change among the three stages. Set to the quantity ΔX1.

  On the other hand, if it is determined in step 207 that the response index Rs of the air-fuel ratio sensor 37 is equal to or less than the second determination value Rr2, the process proceeds to step 208, where the response index Rs of the air-fuel ratio sensor 37 is the first response index Rs. 3 is determined to be greater than the third determination value Rr3, and if the response index Rs of the air-fuel ratio sensor 37 is determined to be greater than the third determination value Rr3, the response of the air-fuel ratio sensor 37 is determined. It is determined that the degree of decrease is moderate, and the routine proceeds to step 210, where the air-fuel ratio change amount ΔX of the air-fuel ratio dither control is set to the second smallest change amount ΔX2 in the three stages.

  If it is determined in step 208 that the response index Rs of the air-fuel ratio sensor 37 is equal to or less than the third determination value Rr3, it is determined that the degree of decrease in response of the air-fuel ratio sensor 37 is large. In step 211, the air-fuel ratio change amount ΔX of the air-fuel ratio dither control is set to the largest change amount ΔX3 among the three stages.

  In steps 207 to 211, the air-fuel ratio change amount ΔX of the air-fuel ratio dither control is changed in three stages according to the degree of decrease in the response of the air-fuel ratio sensor 37. You may make it change with. Further, the air-fuel ratio change amount ΔX of the air-fuel ratio dither control may be continuously changed according to the degree of responsiveness reduction of the air-fuel ratio sensor 37.

  After the air-fuel ratio change amount ΔX of the air-fuel ratio dither control is set in this way, the routine proceeds to step 212 where a dither method cylinder-by-cylinder air-fuel ratio estimation routine of FIG. 6 described later is executed to execute the air-fuel ratio dither control. Based on the output of the air-fuel ratio sensor 37 at this time, the air-fuel ratio of the i-th cylinder #i cylinder which is the current air-fuel ratio estimation target is estimated. The processing in these steps 205 to 212 serves as cylinder-by-cylinder air-fuel ratio estimation means in the claims.

  After estimating the air-fuel ratio of the i-th cylinder #i cylinder in step 206 or step 212, the process proceeds to step 213 in FIG. 5, and the estimated air-fuel ratio is set according to the engine operating state (engine speed, engine load, etc.). After the correction, the routine proceeds to step 214 where the deviation between the estimated air-fuel ratio AF (#i) of the i-th cylinder #i and the reference air-fuel ratio (the average value or the control target value of the estimated air-fuel ratio of all cylinders) is calculated. After calculating the inter-cylinder deviation Δaf (#i) of the air-fuel ratio of the i-th cylinder #i, the routine proceeds to step 215, where the inter-cylinder deviation Δaf (#i) of the air-fuel ratio of the i-th cylinder #i is a predetermined determination value. It is determined whether or not it is larger than F.

  As a result, if it is determined that the inter-cylinder deviation Δaf (#i) of the air-fuel ratio of the i-th cylinder #i is equal to or less than the determination value F, the process proceeds to step 221 and the air-fuel ratio abnormality of the i-th cylinder #i is abnormal. After determining that there is no (normal) and setting the normal flag Xafnorm (#i) of the i-th cylinder #i to “1”, this routine is ended.

  On the other hand, if it is determined in step 215 that the inter-cylinder deviation Δaf (#i) of the air-fuel ratio of the i-th cylinder #i is larger than the determination value F, the process proceeds to step 216 and the i-th cylinder The count value of the delay counter D (#i) of the i-th cylinder #i that measures the elapsed time after the inter-cylinder deviation Δaf (#i) of the air-fuel ratio of #i becomes larger than the determination value F is “1”. Then, the process proceeds to step 217, where it is determined whether the count value of the delay counter D (#i) has exceeded a predetermined delay value, so that the inter-cylinder deviation Δaf (#i) is greater than the determination value F. It is determined whether or not a predetermined delay time has elapsed since the increase.

  In this step 217, the count value of the delay counter D (#i) exceeds a predetermined delay value (a predetermined delay time has elapsed since the inter-cylinder deviation Δaf (#i) is larger than the determination value F). When it is determined that the process proceeds to step 218, the process of incrementing the count value of the abnormality counter T (#i) of the i-th cylinder #i by “1” is started, and then the process proceeds to step 219, where the abnormality counter T ( It is determined whether the count value of #i) exceeds a predetermined abnormality determination value.

  In this step 219, when it is determined that the count value of the abnormality counter T (#i) is smaller than the abnormality determination value, this routine is ended as it is, the engine operating state is in the predetermined operating region, and When the inter-cylinder deviation Δaf (#i) is larger than the determination value F, the process of incrementing the count value of the abnormality counter T (#i) (steps 201 to 218) is repeated. When the engine operating state is not in the predetermined operating range, or when the inter-cylinder deviation Δaf (#i) is less than or equal to the determination value F, the current count value is not incremented without incrementing the count value of the abnormality counter T (#i). Hold.

  Thereafter, if it is determined in step 219 that the count value of the abnormality counter T (#i) has exceeded the abnormality determination value, the process proceeds to step 220 and it is determined that the air-fuel ratio of the i-th cylinder #i is abnormal. The abnormality flag Xaffail (#i) of the i-th cylinder #i is set to “1” and a warning lamp (not shown) provided on the instrument panel of the driver's seat is turned on, or the driver's seat A warning is displayed on a warning display section (not shown) of the instrument panel to warn the driver, and the abnormality information (abnormal code, etc.) is rewritable nonvolatile memory such as a backup RAM (not shown) of the ECU 40. And the routine is terminated.

  On the other hand, before it is determined in step 219 that the count value of the abnormality counter T (#i) has exceeded the abnormality determination value, it is determined in step 215 that the inter-cylinder deviation Δaf (#i) is equal to or less than the determination value F. If YES, the routine proceeds to step 221 where it is determined that the air-fuel ratio of the i-th cylinder #i is not abnormal (normal), and the normal flag Xafnorm (#i) of the i-th cylinder #i is set to “1”. Then, this routine is terminated.

[Dither method cylinder-by-cylinder air-fuel ratio estimation routine]
The dither method cylinder-by-cylinder air-fuel ratio estimation routine shown in FIG. 6 is a subroutine executed in step 212 of the cylinder-by-cylinder abnormality diagnosis routine in FIG. When this routine is started, first, in step 301, air-fuel ratio dither control is executed in which the air-fuel ratio dither control flag forcibly changes the air-fuel ratio of the i-th cylinder #i that is the current air-fuel ratio estimation target. It is determined whether or not it is set to “ON”, which means

  If it is determined in step 301 that the air-fuel ratio dither control flag is off (that is, before the start of air-fuel ratio dither control), the routine proceeds to step 302, where air-fuel ratio dither control is started Is calculated by calculating a deviation Y1 (#i) between the detected air-fuel ratio of the i-th cylinder #i detected by the air-fuel ratio sensor 37 and the reference air-fuel ratio before the air-fuel ratio dither control is started. An inter-cylinder deviation Y1 (#i) of the detected air-fuel ratio of cylinder #i is obtained.

  Thereafter, the routine proceeds to step 303, where air-fuel ratio dither control is executed in which the air-fuel ratio of the i-th cylinder #i is forcibly changed by a predetermined change amount ΔX (#i) in the rich or lean direction. In this air-fuel ratio dither control, for example, by increasing or decreasing the fuel injection amount of the fuel injection valve 20 of the i-th cylinder #i by a predetermined amount, the air-fuel ratio of the i-th cylinder #i is forcibly changed by a predetermined change amount ΔX ( The air-fuel ratio change amount ΔX) set in steps 209 to 211 in FIG. 4 is changed. In this case, the fuel injection amount may be increased or decreased in a constant operation state in which the intake air amount does not change, whereby the air-fuel ratio can be accurately changed by the predetermined change amount ΔX.

In the case of a system in which a throttle valve is provided for each cylinder, the throttle valve opening of the i-th cylinder #i is adjusted to increase or decrease the intake air amount of the i-th cylinder #i by a predetermined amount. Thus, the air-fuel ratio of the i-th cylinder #i may be forcibly changed by a predetermined change amount ΔX. In this case, the intake air amount may be increased or decreased in a constant operating state in which the fuel injection amount of each cylinder does not change, whereby the air-fuel ratio can be accurately changed by the predetermined change amount ΔX.
Thereafter, the process proceeds to step 304, the air-fuel ratio dither control flag is set on, and then the process proceeds to step 305.

  On the other hand, after the air-fuel ratio dither control flag is set on, it is determined in step 301 that the air-fuel ratio dither control flag is on. Therefore, the processing in steps 302 to 304 is skipped and the process proceeds to step 305.

  In this step 305, the air-fuel ratio sensor 37 detects the air-fuel ratio of the exhaust gas of the i-th cylinder #i after the air-fuel ratio dither control is started for a predetermined period (after the air-fuel ratio is forcibly changed). When it is determined that the predetermined period has elapsed, the process proceeds to step 306 and after the start of the air-fuel ratio dither control (after the air-fuel ratio is forcibly changed). By calculating the deviation Y2 (#i) between the detected air-fuel ratio of the i-th cylinder #i detected by the air-fuel ratio sensor 37 and the reference air-fuel ratio, the detected sky of the i-th cylinder #i after the start of the air-fuel ratio dither control is calculated. An inter-cylinder deviation Y2 (#i) of the fuel ratio is obtained.

Thereafter, the process proceeds to step 307, where the actual air-fuel ratio change ΔX (#i) when the air-fuel ratio of the i-th cylinder #i is forcibly changed by the air-fuel ratio dither control, and the air-fuel ratio sensor at that time 37, the change amount ΔY (#i) of detected air-fuel ratio {= Y2 (#i) −Y1 (#i)}, and the inter-cylinder deviation Y1 of the detected air-fuel ratio of the air-fuel ratio sensor 37 before the start of the air-fuel ratio dither control (#i) is used to determine the inter-cylinder deviation X (#i) of the actual air-fuel ratio of the i-th cylinder #i before the start of the air-fuel ratio dither control by the following equation.
X (#i) = ΔX (#i) × Y1 (#i) / ΔY (#i)
= ΔX (#i) × Y1 (#i) / {Y2 (#i) −Y1 (#i)}

  Thereafter, the routine proceeds to step 308, where the actual air-fuel ratio deviation X (#i) of the i-th cylinder #i and the reference air-fuel ratio (the average value or control target value of the estimated air-fuel ratios of all cylinders) are determined. After calculating the air-fuel ratio of the i-th cylinder #i, the routine proceeds to step 309, where the air-fuel ratio dither control is terminated and the air-fuel ratio dither control flag is reset to OFF, and then this routine is terminated.

  In the present embodiment described above, when the responsiveness of the air-fuel ratio sensor 37 is lowered due to deterioration over time or the like, the cylinder-by-cylinder air-fuel ratio estimation method is an estimation method that is less affected by the decrease in responsiveness of the air-fuel ratio sensor 37 (for example, Dither method), even when the responsiveness of the air-fuel ratio sensor 37 is lowered, it is possible to reduce a decrease in the estimation accuracy of the cylinder-by-cylinder air-fuel ratio, and the cylinder using the estimation result of the cylinder-by-cylinder air-fuel ratio. It is possible to reduce a decrease in diagnosis accuracy of another abnormality diagnosis.

  In the present embodiment, when the responsiveness of the air-fuel ratio sensor 37 is within the normal range (a state in which the responsiveness of the air-fuel ratio sensor 37 has hardly decreased), the cylinder-by-cylinder air-fuel ratio estimation method is used as the cylinder-by-cylinder air-fuel ratio estimation method. Since the model method for estimating the air-fuel ratio of each cylinder using the estimation model is selected, when the responsiveness of the air-fuel ratio sensor 37 is within the normal range, the air-fuel ratio of each cylinder is accurately determined by the model method. Can be estimated. Moreover, since the dither method performs air-fuel ratio dither control that forcibly changes the air-fuel ratio for each cylinder, there is a possibility that drivability and exhaust emission may be reduced, but the model method forcibly sets the air-fuel ratio. Therefore, it is possible to accurately estimate the air-fuel ratio of each cylinder without deteriorating drivability and exhaust emission.

  On the other hand, when the responsiveness of the air-fuel ratio sensor 37 falls below the normal range, the air-fuel ratio when the air-fuel ratio dither control for forcibly changing the air-fuel ratio for each cylinder is executed as the cylinder-by-cylinder air-fuel ratio estimation method. Since the dither method for estimating the air-fuel ratio of each cylinder is selected based on the output of the sensor 37, the air-fuel ratio of each cylinder can be estimated with higher accuracy than the model method, and the responsiveness of the air-fuel ratio sensor 37 can be estimated. Even when the air pressure decreases, it is possible to reduce the decrease in the estimation accuracy of the cylinder-by-cylinder air-fuel ratio.

  In this embodiment, when the dither method is selected as the cylinder-by-cylinder air-fuel ratio estimation method, the air-fuel ratio change amount of the air-fuel ratio dither control is set according to the degree of decrease in the response of the air-fuel ratio sensor 37. Therefore, the air-fuel ratio change amount of the air-fuel ratio dither control is changed according to the degree of decrease in the responsiveness of the air-fuel ratio sensor 37, so that the estimation accuracy of the cylinder-by-cylinder air-fuel ratio is appropriately secured, and drivability and exhaust emission are achieved. The air-fuel ratio change amount of the air-fuel ratio dither control can be set so as to minimize the adverse effect exerted.

  Note that the cylinder-by-cylinder air-fuel ratio estimation method and the cylinder-by-cylinder abnormality diagnosis method are not limited to the methods described in the above embodiments, and may be changed as appropriate. For example, in a system in which one or both of the intake valve 25 and the exhaust valve 26 are electromagnetically driven valves (valve devices that can be opened and closed independently for each cylinder), the responsiveness of the air-fuel ratio sensor 37 is higher than the normal range. In the case of a decrease, the air-fuel ratio of each cylinder may be estimated by the specific cylinder deactivation method.

  In this specific cylinder deactivation method, during the fuel cut period at the time of deceleration, except for one of the four cylinders of the engine 11, the fuel injection and intake / exhaust valves 25 and 26 are deactivated and the cylinders are deactivated. With the intake / exhaust port closed, fuel injection and intake / exhaust valves 25 and 26 are opened / closed only for the remaining one cylinder. In the following description, a cylinder that pauses the opening / closing operation of the fuel injection and intake / exhaust valves 25, 26 is referred to as a “pause cylinder”, and a cylinder that performs the opening / closing operation of the fuel injection / intake / exhaust valves 25, 26 is “operated”. Called "cylinder". During the fuel cut period during deceleration, the operating cylinders are sequentially switched one by one every predetermined time, and the air-fuel ratio of the operating cylinders is detected based on the output of the air-fuel ratio sensor 37 each time. In this case, all the remaining cylinders except for one cylinder are set as idle cylinders, and the intake / exhaust ports of these idle cylinders are closed, and the fuel injection and intake / exhaust valves 25 and 26 are opened and closed only for one operating cylinder. Since the operation is executed, the air-fuel ratio of the operating cylinder can be accurately detected based on the output of the air-fuel ratio sensor 37.

  Further, in the above embodiment, in the system that performs the abnormality diagnosis for each cylinder that determines the presence / absence of abnormality in each cylinder based on the estimation result of the air-fuel ratio for each cylinder, the air-fuel ratio for each cylinder is determined according to the responsiveness of the air-fuel ratio sensor 37. Although the estimation method is switched, a system for performing cylinder-by-cylinder air-fuel ratio control for controlling the air-fuel ratio of each cylinder so as to reduce the variation between cylinders in the air-fuel ratio of each cylinder based on the estimation result of the cylinder-by-cylinder air-fuel ratio The cylinder-by-cylinder air-fuel ratio estimation method may be switched in accordance with the responsiveness of the air-fuel ratio sensor 37.

In the above embodiment, the present invention is applied to a four-cylinder engine. However, the present invention may be applied to a two-cylinder engine, a three-cylinder engine, or an engine having five or more cylinders.
In addition, according to the present invention, the cylinder-by-cylinder air-fuel ratio estimation method may be switched according to the responsiveness of the air-fuel ratio sensor 37 from among three or more types of cylinder-by-cylinder air-fuel ratio estimation methods.

It is a schematic block diagram of the whole engine control system in one Example of this invention. (A) And (b) is a figure for demonstrating the air-fuel ratio estimation method according to cylinder by the dither method. It is a flowchart explaining the flow of a process of a sensor abnormality diagnosis routine. It is a flowchart (the 1) explaining the flow of a process of the abnormality diagnosis routine according to cylinder. It is a flowchart (the 2) explaining the flow of a process of the abnormality diagnosis routine classified by cylinder. It is a flowchart explaining the flow of a process of the air-fuel ratio estimation routine according to cylinder of a dither method.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 11 ... Engine (internal combustion engine), 12 ... Intake pipe, 15 ... Throttle valve, 20 ... Fuel injection valve, 35 ... Exhaust manifold, 36 ... Exhaust junction, 37 ... Air-fuel ratio sensor, 40 ... ECU (air-fuel ratio estimation for each cylinder) Means, response detection means, cylinder-specific abnormality diagnosis means)

Claims (5)

An air-fuel ratio sensor for detecting the air-fuel ratio of the exhaust gas is installed at an exhaust gas merging portion where the exhaust gases of a plurality of cylinders of the internal combustion engine merge, and the air-fuel ratio of each cylinder is estimated based on the detection value of the air-fuel ratio sensor In the control device for an internal combustion engine provided with the cylinder-by-cylinder air-fuel ratio estimating means,
Responsiveness detecting means for detecting the responsiveness of the air-fuel ratio sensor,
The cylinder-by-cylinder air-fuel ratio estimation means has a plurality of types of cylinder-by-cylinder air-fuel ratio estimation methods for estimating the air-fuel ratio of each cylinder based on the detection value of the air-fuel ratio sensor, and the air-fuel ratio detected by the responsiveness detection means A control apparatus for an internal combustion engine, wherein the cylinder-by-cylinder air-fuel ratio estimation method is switched according to the response of the sensor.   The cylinder-by-cylinder air-fuel ratio estimation means uses the detected value of the air-fuel ratio sensor and the air-fuel ratio of each cylinder as the cylinder-by-cylinder air-fuel ratio estimation method when the responsiveness of the air-fuel ratio sensor is within a predetermined normal range. 2. The control apparatus for an internal combustion engine according to claim 1, wherein a model method for estimating an air-fuel ratio of each cylinder is selected using an associated model.   The cylinder-by-cylinder air-fuel ratio estimating means is an air-fuel ratio estimating method for forcibly changing the air-fuel ratio for each cylinder when the responsiveness of the air-fuel ratio sensor falls below a predetermined normal range. 3. The control device for an internal combustion engine according to claim 1, wherein a dither method for estimating an air-fuel ratio of each cylinder is selected based on an output of the air-fuel ratio sensor when fuel ratio dither control is executed.   When the dither method is selected as the cylinder-by-cylinder air-fuel ratio estimation method, the cylinder-by-cylinder air-fuel ratio estimation means forcibly sets the air-fuel ratio by the air-fuel ratio dither control according to the degree of decrease in the response of the air-fuel ratio sensor. 4. The control apparatus for an internal combustion engine according to claim 3, wherein an air-fuel ratio change amount at the time of changing to is set.   5. A cylinder-specific abnormality diagnosis unit that performs cylinder-specific abnormality diagnosis for determining whether or not each cylinder is abnormal based on the estimation result of the cylinder-by-cylinder air-fuel ratio. The internal combustion engine control device described.

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